No One Really Knows Why—Is This True? (Science Geeky)

C

cemusic

Inspired
I had a student ask me a question along these lines a while back, and since then have been asking friends who are luthiers, physics geeks, whatever, and the consensus seems to be that no one is really sure. It seems like there are lots of science and engineering folks around here so…

Why do different steel stringed instruments sound so much like “themselves” when amplified using magnetic pickups?. A magnetic pickup only responds to electromagnetic vibrations, (mostly) generated by a nearby metal string which, as far as we know, are not affected by korina guitar bodies, spruce tops, tone chambers—whatever. So that means that the vibration of the string itself is somehow affected enough by the acoustic quality of the instrument so as to have a huge effect upon what comes out of the amp.

I think anyone who has any experience with electrified guitars can agree that a Martin acoustic and a Les Paul, amplified with identical magnetic pickups, will not only sound radically different from each other, they will retain at least some of their own acoustic character when run through an amp.

Where/how does all of this ambient sonic information get transmitted to the string/pickup? Phase cancellation transmitted to the string through the air? What else is there?

There is lots of BS and half baked guesswork on various forums out there. If anyone here has any specific knowledge, or even knows of a place to do further reading, I would be grateful.

BTW my Google searches yielded some pretty funny results, e.g. I found lots of acoustic guitar tab for a band called “Magnetic Fields” :)
 
I'll take a shot ... My guess would be that it has to do with the damping/enhancement of the overtone series generated by the string as it interacts with the rest of the guitar. As you probably know, when you strike a string you generate not only the fundamental vibration of the string but also it's overtone series. It's the overtone series that makes an E on one instrument sound different than an E on another instrument - whether that be two different guitars, or piano or sax. Now the string, bridge, and guitar body and other components all interact with one another and it's possible that this interaction changes the overtone series of the vibrating string. Some overtones may be enhanced others may be reduced. So keep every thing else the same and change the body of the guitar and you get a different overtone series and therefore a different sound. Take this with a grain of salt .. hypothesis at this point, could be wrong :)
 
The density and type of wood, as well as the combination are going to affect the way the strings vibrate. Just recently I went from a rosewood board back to a maple neck on my tele and the tone is significantly brighter now, I don't know why a denser wood like maple has a brightening effect per se but it's definitely there. Maybe rosewood absorbs more of the vibrational energy from the strings and it translates to warmer tones? I don't know
 
Yep, the scale length, pickup position, and wood resonances affect the vibration and thereby the harmonic power distribution and decay.
If you stick your ear on the wood of an electric it is actually very loud. Probably has the same or greater effect on string tone and decay than a loud amp in proximity has - just guessing about that part..
 
Every point of contact of the string vibration will generate mechanical energy transfer through the various mediums with which the string interacts. Each of these systems feeds back into the string generating various overtones and harmonics as well as more random vibrations which all add up to "Timbre". The string vibration is no longer composed only of a fundamental and so the mediums involved - "tone"-woods, brass, steel, chrome, nickel, etc - all impart characteristics to the string vibration.
The pickup mounts themselves affect the vibration or lack of it of the pickup itself and so the interaction of the pickup and string is also variable depending on whether the guitar top itself resonates.
Then when amplified, the sound feedbacks into the instrument - string and body - to some degree and so the systems get even more "complex".

It's really basic physics, nothing mysterious about it.
 
cemusic said:
Why do different steel stringed instruments sound so much like “themselves” when amplified using magnetic pickups?
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Two reasons:

1) When your guitar's body is excited by a vibrating string, it vibrates in a way that's affected by it's own resonances—its own EQ. Your pickups are mounted to...your guitar's body. Which is vibrating. Your pickups go along for the ride, moving with the wood's vibrations. They have to, because the base they're mounted on is vibrating. Than means the pickup is moving relative to the strings, and that movement is going change the string movement that your pickup "sees."

2) All pickups are microphonic to some extent, a you can see and hear in the video that @Toopy14 posted above.
 
The vibration characteristics of the strings are dependent upon the size, shape, material, etc. of the body along with a host of other things. These instruments characteristics imprint themselves onto the vibration pattern of the strings which affects the relative amplitudes (and phase) of the harmonics and the envelope of those harmonics. The pickups simply (more or less) translate the vibration of the strings into an electric current.

The instrument is resonant so when you pluck a string certain harmonics are emphasized. The type of wood, shape, etc. affects the frequency and Q of the various resonances which, in turn, affects which harmonics are emphasized and and their decay characteristics. You can think of a stringed instrument as a formant synthesizer. There are dozens (hundreds?) of resonances at various frequencies with differing Q's. The vibrating string generates acoustic energy by displacing air molecules. The intensity of the sound is proportional to the string displacement. The vibrating string also induces a current in the pickup that is very similar since the current is proportional to the string displacement.
 
FractalAudio said:
The vibration characteristics of the strings are dependent upon the size, shape, material, etc. of the body along with a host of other things. These instruments characteristics imprint themselves onto the vibration pattern of the strings which affects the relative amplitudes (and phase) of the harmonics and the envelope of those harmonics. The pickups simply (more or less) translate the vibration of the strings into an electric current.

The instrument is resonant so when you pluck a string certain harmonics are emphasized. The type of wood, shape, etc. affects the frequency and Q of the various resonances which, in turn, affects which harmonics are emphasized and and their decay characteristics. You can think of a stringed instrument as a formant synthesizer. There are dozens (hundreds?) of resonances at various frequencies with differing Q's. The vibrating string generates acoustic energy by displacing air molecules. The intensity of the sound is proportional to the string displacement. The vibrating string also induces a current in the pickup that is very similar since the current is proportional to the string displacement.
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He got to it first
 
"...but wood doesn't matter in electric guitars." :p

Everything matters to some degree. Even down to how you hold the guitar and how thick and hard your finger calluses are will affect the vibration and resonance of the strings and neck/body to some degree. Armchair physicists tend to drastically over-simplify complex real world systems they may not fully understand. There is usually far more at play than the textbook idealized models might lead people to believe.
 
FractalAudio said:
The vibration characteristics of the strings are dependent upon the size, shape, material, etc. of the body along with a host of other things. These instruments characteristics imprint themselves onto the vibration pattern of the strings which affects the relative amplitudes (and phase) of the harmonics and the envelope of those harmonics. The pickups simply (more or less) translate the vibration of the strings into an electric current.

The instrument is resonant so when you pluck a string certain harmonics are emphasized. The type of wood, shape, etc. affects the frequency and Q of the various resonances which, in turn, affects which harmonics are emphasized and and their decay characteristics. You can think of a stringed instrument as a formant synthesizer. There are dozens (hundreds?) of resonances at various frequencies with differing Q's. The vibrating string generates acoustic energy by displacing air molecules. The intensity of the sound is proportional to the string displacement. The vibrating string also induces a current in the pickup that is very similar since the current is proportional to the string displacement.
Click to expand...

This and the video from Toopy14 make me think that the strings--not just the vibrating at the moment string--all of the strings act as, I guess, transducers. I you knock on a magnetic equipped guitar body and hear a knock out of the amp, isn't that just because the vibrations have been transferred from the guitar body to the strings to the pickup? And even if the strings have been deadened so they don't resonate at pitch, it seems like they function as a 'diaphragm' (as in a mic) that interacts with the pickup.

Does that mean that the stronger the bridge and strings are coupled to the body, the more the resonant characteristics are transferred to the pickups?

Just thinking outloud....
 
cemusic said:
This and the video from Toopy14 make me think that the strings--not just the vibrating at the moment string--all of the strings act as, I guess, transducers.
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The strings are part of a transducer. The pickups are the other part.


cemusic said:
I you knock on a magnetic equipped guitar body and hear a knock out of the amp, isn't that just because the vibrations have been transferred from the guitar body to the strings to the pickup?
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It's not just because of that. Take another look at the video @Toopy14 posted. There are no strings involved.
 
Rex said:
The strings are part of a transducer. The pickups are the other part.



It's not just because of that. Take another look at the video @Toopy14 posted. There are no strings involved.
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Wow yeah, i guess i kind of FFWD'ed past the part about no strings and didn't notice-my bad--but at the end he's tapping on the vibrato springs, which I would have to believe behave very much like the strings with regards to what the electromagnetic pickup sees.
So I guess it seems to me that we have all this metallic structural stuff attached to and around the pickup that has varying transducer effects that are completely disregarded. Kind of like having a studio space without regard to acoustic design?
 
cemusic said:
Wow yeah, i guess i kind of FFWD'ed past the part about no strings and didn't notice-my bad--but at the end he's tapping on the vibrato springs, which I would have to believe behave very much like the strings with regards to what the electromagnetic pickup sees.
So I guess it seems to me that we have all this metallic structural stuff attached to and around the pickup that has varying transducer effects that are completely disregarded. Kind of like having a studio space without regard to acoustic design?
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And did you notice the part where he completely removed the pickup from the body yet you could still hear the knocks on the guitar? now that don't make no sense. How is the guitar picking up vibrations from something it is no longer attached to?
 
cemusic said:
This and the video from Toopy14 make me think that the strings--not just the vibrating at the moment string--all of the strings act as, I guess, transducers.
Click to expand...


Hi my friend, I do not have high technical knowledge but I like to use the right words ... I see a transducer as a kind of translator: a device that converts one type of energy into another. The electromagnet of the pickup converts vibrations to voltage as well as the electromagnet of a speaker acts in reverse.


I do not see the strings (neither the "metallic structural stuff") as transducer in a strict sense but as a receiver / emitter ... a vehicle. A vehicle that provides its own character as part of a complex system.


I see guitar as a complex and dynamic system, like an ecosystem. In nature ecosystems are composed of living beings and inert elements that interact. The guitar acts as a single resonant body, as an unit, but if you zoom in and see what happens in detail, what happens in the bridge, in the nut, the body, the angles, the distances... things go wild!


If you oversimplify this system you lose many nuances so please don’t say mostly!


You can look at the string as the beginning of the events chain, but things are not that simple. By striking the string its vibration expands towards the rest of the elements of the guitar and ends up becoming modified.


In the same way you can see the pickup as the end, but neither is it so simple. The magnetism of the pickup also interacts with the string movement at the beginning of the chain of events (even unplugged).


We usually call acoustic only one type of guitar but really all guitars are acoustic. When the body is solid the resonance is smaller, but it is still there ... let me explain it in another way.


At the beginning of the twentieth century when the amplification began, the musicians experienced feedback on their hollow body guitars (we could say that the problem was "too much resonance"???), they tried to mitigate it by filling the body with clothes, newspapers ... although the definitive solution was the implantation of semi-hollow and solid bodies.


That is to say, an electric guitar has its resonance "mitigated" in order to obtain a practical use when connecting it to an amplifier, but it still has his own character, his timbre. Our ears acts as a mass spectrometer and are able to receive and process minute details that allow us to distinguish between a rosewood and a maple fingerboard.

The capture of the acoustic timbre by the pickup and the following amplification are not fully efficient and adds more complexity to the resultant tone, that's why Cliff sais that the vibrating string induces a current in the pickup that is very similar (but not exactly the same).

If you knock the body of a fixed bridge electric guitar without strings, you still hear the knock on the amplifier and there are no springs involved. As Rex says all the pickups are microphonic.
 
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My guess is that wood makes a lot less of an impact than most people say. However I'll offer a hypothesis about why wood *might* actually matter a tiny bit to the sound of an electric guitar.

The assumption seems to be that the pickups are fixed while only the string vibrates, but this can't be true. The wood body clearly does resonate and thus the pickups are not a stationary reference point. If any effect on body resonance is measurable in the pickup output then its possibly due in part to the relative motion of the pickup and string that alters the signal being generated from the theoretical ideal. There is also the microphonic interference of the pickup where the resonance directly induces a signal in the coils as demoed in the video above.

Which of the two is more important, who knows but I *suspect* that the string's vibration itself is not being modified much if at all by the material of the guitar. I'd love to see a rigorous, scientific, fully blinded, controlled test done to put this one to bed but I doubt anyone would be willing to put the effort in.
 
After recovering from my head exploding by the video that @Toopy14 showed. I have some thoughts which are probably not even close to being correct. :)

Is seems to me that the act of hitting that stringless guitar is somehow altering the existing electromagnetic field and causing interference. This in turn is being amplified (by an incredible degree in this case). I wonder if things like this are at play here:

https://en.wikipedia.org/wiki/Permeability_(electromagnetism)

On a side note, I'm not sure this really affects much of the character of the tone because my guess it's being greatly amplified to prove a point. I would think the other aspects already mentioned above are what are really changing the characteristics of the tone.
 
massless said:
My guess is that wood makes a lot less of an impact than most people say.
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There are some stone guitars, but I find that something is missing...it also weighs a lot hahaha!



There's a cardboard guitar, still an organic material, but I don't buy it
 
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